1. Field of the Invention
The invention relates primarily to the field of charge-coupled devices, particularly charge-coupled devices used in shift registers and memories.
2. Prior Art
Charge-coupled devices (CCD) have been known for a number of years and have found applications, for example, in the fields of memory storage and video displays ("Charge-Coupled Devices - A New Approach to MIS Device Structures", IEEE Spectrum, July 1971, by W. S. Boyle and G. E. Smith, beginning on page 18). In the memory field, particularly where metal-oxide-silicon (MOS) technology is employed, memory storage is feasible where a plurality of shift registers are fabricated on a silicon substrate. An analysis of anticipated cost of such CCD devices, in comparison to TTL RAMs and other semiconductor memories and other memory storage means such as magnetic disks may be found in an article entitled "Charge-Coupled Devices Move in on Memories and Analog Signal Processing", Electronics, Aug. 8, 1974, by Laurence Altman, beginning on page 91.
In order to mass produce a CCD memory one problem that must be solved is the reliable detection of the output from a CCD device both for reading information from the memory and for refreshing it. Typically, the charge output of any particular CCD device will vary from one production lot to another. For example, CCD devices are known to be particularly sensitive to variations in device dimensions which will occur in normal masking operations. Also, at high temperatures and/or at low operating frequencies, thermal generation of carriers can significantly affect the CCD output. Moreover, to achieve low cost, high density smaller devices are required which make signal level detection even more difficult.
In order to fabricate a reliable, low cost CCD memory, some "on-chip" self adjustment or compensation must be provided to compensate for the above-described variations. The present invention provides a self-regulating signal which adjusts the threshold of the detection circuitry so that the latter correctly senses the output of a storage charge-coupled shift register. The disclosed system provides built-in compensation which enables reliable and effective sensing despite variations due to processing or operating frequency, temperature or voltage.